Depressed participants were found to be younger and more likely to smoke tobacco than healthy participants. The prevalence of smoking among depressed women was higher and they smoked more ( an average of 15 cigarettes per day) than women in the control group. Physical activity was shown to protect against depression; however, the association between smoking and depression was not explained by differences in physical activity. During the study period, 87 of the 781 smoking women developed depression. In contrast, among 584 non-smoking women, only 38 developed depressive disorders.

The study, according to the researchers, provides evidence consistent with the hypothesis that smoking is associated with depression. The data presented suggest that smoking is associated with a twofold increased risk of depression over the 10-year follow-up period. This association was not associated with age or physical activity or alcohol consumption. The authors cite a Norwegian study in which, over an 11-year follow-up period, the proportion of first-onset depression increased with smoking; for example, smokers who smoked the most cigarettes had a risk of depression four times higher than non-smokers. Other studies have provided support for a further thesis that depression and smoking coexist as a phenomenon with a common cause, for example genetic. The efficacy of bupropion ( bupropion), used to treat both depression and nicotine dependence, may indicate a commonality between the two conditions at the neurochemical level.

The authors of the present study believe that the data they collected strongly indicate that smoking has an adverse role in depression and that more efforts including smoking as a target for routine interventions are needed.